Wireless systems typically upconvert a baseband signal to a Radio Frequency (RF) signal for transmission, and downconvert received RF signals to baseband for processing. Such frequency conversion requires producing a reliable mixing frequency signal, typically referred to as a local oscillator (LO) signal, for use in the RF front-end of a wireless device. Phase-Locked Loops (PLLs) are often used to provide such mixing frequency signals.
In some cases, stringent requirements are placed on the mixing frequency signal, such as produced by a PLL. For example, it is foreseen that 5G cellular systems will use millimeter waves, where the frequencies currently in discussion range between 15 GHz and 60 GHz. In order to use such 5G system outdoors, a longer cyclic prefix has to be used compared to newly released 60 GHz indoor systems. Such longer cyclic prefixes necessitate a closer sub-carrier spacing in the OFDM modulation. This closer sub-carrier spacing poses stringent phase noise requirements on the outputs of the PLLs. At the same time, beamforming should be supported to increase the range and capacity of the system, which results in a large number of antenna elements. The signal at each antenna element of a beamforming system will have an individual phase shift that controls the beam direction. In some implementations, the beam controlling phase shifts are imposed on the mixing signal. In any event, accurate beamforming requires accurate phase shifts. It is also desirable to be able to program the frequency of the mixing signal to enable the wireless device to operate on different frequency channels and in different bands.
As a result of all of these considerations, there is a need to improve the generation of the mixing frequency signals so as to provide the desired frequency programmability, to provide the desired phase control, and to provide improved phase noise performance, particularly in light of possible future 5G systems.